pay attention to EMSGSIZE errors when sending pmtu verification packets

and drop the pmtu accordingly
This way, when we actually *do* get pmtu info from the network/kernel
we can discover the correct pmtu much more quickly. Especially in the
absence of other traffic, forwarding of which was previously the only
place where we'd check for EMSGSIZE.

Fixes #12

forwarder.go

@@ -70,15 +70,29 @@ func (conn *LocalConnection) forwarderLoop(forwardCh <-chan *ForwardedFrame, sto
 	var pmtuVerifyTick <-chan time.Time
 	var clientPMTU int
 	maxPayload := pmtu - UDPOverhead
+	calculateClientPMTU := func(ftbe FrameTooBigError) {
+		maxPayload = ftbe.PMTU - UDPOverhead
+		clientPMTU = maxPayload - enc.PacketOverhead() - enc.FrameOverhead() - EthernetOverhead
+	}
 	updateClientPMTU := func() {
-		conn.setClientPMTU(clientPMTU)
-		pmtuVerifyFrame := &ForwardedFrame{
-			srcPeer: conn.local,
-			dstPeer: conn.remote,
-			frame:   make([]byte, clientPMTU+EthernetOverhead)}
-		for cnt := 0; cnt < 10; cnt += 1 {
-			enc.AppendFrame(pmtuVerifyFrame)
-			udpSender.Send(enc.Bytes())
+		for gotNewPMTU := true; gotNewPMTU; {
+			conn.setClientPMTU(clientPMTU)
+			pmtuVerifyFrame := &ForwardedFrame{
+				srcPeer: conn.local,
+				dstPeer: conn.remote,
+				frame:   make([]byte, clientPMTU+EthernetOverhead)}
+			gotNewPMTU = false
+			for cnt := 0; cnt < 10; cnt += 1 {
+				enc.AppendFrame(pmtuVerifyFrame)
+				err := udpSender.Send(enc.Bytes())
+				if err != nil {
+					if ftbe, ok := err.(FrameTooBigError); ok {
+						calculateClientPMTU(ftbe)
+						gotNewPMTU = true
+						break
+					}
+				}
+			}
 		}
 		pmtuVerifyTick = time.After(PMTUVerifyTimeout)
 	}
@@ -94,8 +108,7 @@ func (conn *LocalConnection) forwarderLoop(forwardCh <-chan *ForwardedFrame, sto
 		err := udpSender.Send(enc.Bytes())
 		if err != nil {
 			if ftbe, ok := err.(FrameTooBigError); ok {
-				maxPayload = ftbe.PMTU - UDPOverhead
-				clientPMTU = maxPayload - enc.PacketOverhead() - enc.FrameOverhead() - EthernetOverhead
+				calculateClientPMTU(ftbe)
 				updateClientPMTU()
 			} else if PosixError(err) == syscall.ENOBUFS {
 				// TODO handle this better